4.4 TMS-03 Ramp Metering

4.4.1 ITS service at a glance

4.4.2 ITS service profile

4.4.2.1 General ITS service description

During peak or congested periods on the motorway, the addition of traffic from on-ramps causes vehicles to break or change lanes giving rise to higher occupancy and lower headways. Shorter headways cause drivers to reduce their speeds, resulting in a sustained loss of throughput. 

This speed reduction often causes following vehicles to brake, resulting in a propagation wave of slowing vehicles that travels back along the line of traffic on the main carriageway upstream of the on-ramp. This speed adjustment can occur over a distance of up to 2 km prior to the on-ramp. During this time more vehicles will be attempting to join the main carriageway, and if vehicles continue to join, the speed on the main carriageway will fall to a point where flow breakdown occurs.  Additionally, during peak periods when congestion is increased there may also be a higher risk of accidents. 

Ramp metering (RM) is implemented via the installation of traffic signals on the on-ramps which regulate the flow of traffic joining the motorway during peak or congested periods. It does this by controlling the discharge of vehicles from the on-ramp, holding vehicles back and breaking up onramp platoons, thus reducing the interference of merging vehicles and helping maintain the flow of traffic on the main carriageway. 

The traffic signals are generally operated in dependence of the currently prevailing traffic conditions on both the main carriageway and the on-ramps. 

4.4.2.2 What is the vision?

From the operational perspective, the vision for ramp metering is effective control of onramp vehicles during congested periods, resulting in fewer accidents and maximised mainline throughput. If properly configured and controlled, the on-ramp control has minimal (and controlled) impact on the adjacent road network.

For drivers, encountering ramp metering in an unfamiliar area (i.e. another country) there would not be cause for anxiety as it has a similar look and feel; the driver knows what to expect and how to proceed. Drivers would accept that the small delay experienced on the on-ramp will mean safer, less congested conditions on the motorway.

There is sometimes a need for driver education where the use of ramp metering is new to a region, but essentially the ramp metering relies on the driver obeying mandatory traffic signals.

4.4.2.3 What is the mission?

The purpose of ramp metering is to prevent or delay the onset of flow breakdown on the main carriageway, maximising throughput, without disrupting the urban road network. 

This is achieved by:

• Regulating the flow of additional traffic onto the motorway that, if unregulated would trigger flow breakdown / lead to critical bottlenecks,
• Monitoring and managing the traffic flow on the on-ramp, achieving even distribution, to avoid large platoons of vehicles entering the main carriageway and causing flow breakdown, 
• Reduction or avoidance of congestion spillback to the adjacent urban traffic network or to other merging motorways.
• Ramp metering can also be used to deter drivers making short trips on the motorway and use the urban roads instead. Ramp metering is not used directly to deter drivers making short trips but can have the added benefit that it will discourage drivers who do make short trips from using the motorway network. Coordination with other road operators, urban authorities etc. is required.

Pre-signs and distinct traffic signal heads are used to indicate to drivers there is ramp metering in operation. Nevertheless, the systems rely on the driver obeying mandatory traffic signals as would be the case at any signalised junction.

4.4.2.4 Distinctiveness to other ITS services

Ramp metering is a traffic management measure designed to reduce the disruption from platoons of vehicles entering the main carriageway at on-ramps. The measure is characterised by:

• mainline and ramp traffic monitoring and control
• regulation of the flow of traffic entering the main carriageway via traffic signals positioned on the on ramp
• the use of algorithms to determine the required flow and thus the traffic signal timings
• the presence of traffic signal controls at locations, which might be seen as unusual by some drivers as there is no immediate evidence of traffic conflict as would usually be associated with signalised junctions. 
• sometimes, decentralised control rather than central control from a Traffic Management Centre  

Relevant complementary information, not included within this ITS service and covered by other ITS services, is:

• TTIS-01 and TTIS-02 pre-trip and on-trip Traveller Information: Information dissemination techniques employed by other motorway management functions may be used to inform to motorists en-route or pre-trip about the current operational status of ramp meters.
• TMS-06 Incident Warning and Management: Surveillance and incident warning systems can be used to determine and adjust ramp operational conditions. Data from detectors on the ramp or main carriageway can be used to adjust ramp metering parameters. CCTV can be used to verify that ramp meters are functioning optimally or to observe the effects of ramp metering on traffic flow. Incident management procedures and plans may be integrated with ramp metering to improve safety and restore operations on ramps and the main carriageway in a more timely fashion.  Through active management of ramp meters, and other devices, operators may monitor motorway conditions during emergencies and clear on-ramp queues to allow a faster response to emergencies.
• TMS-04 Hard Shoulder Running and TMS-01 Dynamic Lane Management: Hard shoulder running and dynamic lane management may be used to direct motorists to use certain lanes and to merge out of other lanes. Ramp management strategies can be used in conjunction with lane use controls to manage the demand, leading to motorway sections where lane use controls are active.

4.4.3 Harmonization requirements and advice

4.4.3.1 ITS service definition

Ramp metering is implemented via the installation of traffic signals on the on-ramps which regulate the flow of traffic joining the motorway during peak or congested periods. It does this by controlling the discharge of vehicles from the on-ramp, holding vehicles back and breaking up onramp platoons, thus reducing the interference of merging vehicles and helping maintain the flow of traffic on the main carriageway. The traffic signals are generally operated in dependence of the currently prevailing traffic conditions on both the main carriageway and the on-ramps. 

4.4.3.2 Functional requirements and advice

4.4.3.2.1 Functional architecture

The architecture for ramp metering service is made up of the following three functions: 

• Monitoring of traffic situation on main carriageway (upstream and downstream of access point) and on-ramp,
• algorithms to monitor and control the release rate and
• traffic signals to release the on-ramp traffic.

Figure 61 shows a simple situation together with three data communications interfaces which includes the potential for C-ITS service connectivity in the future. Importantly it illustrates the need to interface with a National Access Point in order to meet the obligations of the ITS Directive.

Cooperation with the adjacent network operators and their traffic control systems also need to be considered where appropriate. This can be achieved using a DATEX II reference profile.

4.4.3.2.2 Functional requirements and advice

• FR1: Automatic and semi-automatic systems should contain models and algorithms that calculate the traffic performance characteristics and transmit it to the ramp metering controller for activation as determined. These models and algorithms can be implemented in a central control system or at the roadside. 
• FR2: The systems should have predefined handling of situations like power failure, disruptions in communications and other functional problems to avoid functional inconsistencies in the service. System parameters and error states should be disseminated in real-time to on duty staff.
• FR3: Traffic Management Centre Operators (or other dedicated resources) must be trained to supervise and manage the systems.
• FR4: When ramp metering interacts with other services such as hard shoulder running, or adjacent ramp metering systems, interfaces should be implemented either at roadsides or in central control systems.
• FR5: Sensors must be adapted to the service and give input to the control system. Note: Systems may include both manual and automatic functions as well as several types of sensors. This requires well defined hierarchical rules and priorities. 
• FR6: Detector data updating frequency should be set correctly in order for the Ramp Metering System to receive enough quality data for the system to operate effectively and in the right circumstances.
• FR7: The traffic signals must display the stop/go light, that the control system has requested.
• FR8: Ramp metering using fixed release time should allow one vehicle per lane to pass during each release phase (green phase).
• FR9: When the signals are connected to are central control system, signals should report actively if activation was successful or not and send error messages if errors occur. Functionality (health) of the system should be monitored continuously.
• FR10: When connected to a central control system, Ramp metering should log all data about the performance of the technical systems and the impact on traffic flow and speed.

Functional advice:

• To be sure that the system is generating the required benefits, measures should be conducted within certain intervals.

4.4.3.3 Interface requirements

Interface requirements:

• IFR1: If the Ramp Metering service provides ramp metering information at interface 1 (see Figure 61), it must provide coded information including the following elements:
  • Location of the ramp metering, either as a linear section or as a group of points (i.e. the effected ramps)
• IFR2: If interface 2 is implemented, the Ramp Metering Service must provide at interface 2 (see Figure 61) Ramp Metering information coded in C-ITS messages including the following elements: 
  • Signal phase
  • Location of the ramp metering
• IFR3: When relevant, the Ramp Metering Service should collect at interface 3 (see Figure 61) C-ITS coded Probe Vehicle Data information such as travel speed, direction, current location of a vehicle (microscopic traffic situation) relevant to this ITS Core service.

4.4.3.4 Organisational requirements and advice

Organisational requirements:

• OR1: Inter- and Intra-Agency Coordination – agreements and cooperation should be established between all authorities / operators when implementing ramp metering from one network to another (e.g. city authority urban roads to motorways or from one regional operator to another).
• OR2: Public Information Campaign – a formal public information campaign should be undertaken in areas where ramp metering is new.

4.4.3.5 Common look & feel requirements and advice

Common Look & feel requirements:

• CL&FR1: Ramp metering traffic signals should be positioned sufficiently far from the merging point to ensure drivers can accelerate enough to reach the speed of the main carriageway and to maximise the storage space on the on-ramp
• CL&FR2: The display of signs/pictograms on VMS or other end-user devices should be in accordance with prevailing national road codes and:
  • Member States which ratified the Vienna Convention MUST respect the Vienna Convention and the European agreement supplementing the convention (1st May 1971) and SHOULD consider the Consolidated Resolution on Road Signs and Signals (R.E.2).
  • Member States which did not ratify the Vienna Convention SHOULD follow the Vienna Convention and also consider the R.E.2.

It is up to the deploying road operator to ensure that real signs are well and widely understood by the road users

• CL&FR3: At least one set of traffic signals should be installed per lane
• CL&FR4: Ramp metering traffic signals should be installed at the metered on-ramps
• CL&FR5: Traffic signals may additionally be installed conform to national regulations
• CL&FR6: Fixed or variable warning pre-signs should be installed on the on-ramp sufficiently upstream of the traffic lights or the on-ramp entrance

• CL&FR7: The traffic signals should operate a “Green – Amber – Red” cycle
• CL&FR8: Ramp metering signals should be distinguishable from regular junction signals.
• CL&FR9: At locations where ramp metering is applied, the number of vehicles released should be communicated to the driver using static information panels stating: “ ‘x’ vehicle per green”.

Common look & feel advice:

• It is advised to install a contrasting yellow shield behind the traffic signals (Figure 63), as used in many European countries.

4.4.3.6 ICT Infrastructure requirements and advice

Infrastructure advice

System Architecture:

• It is advised that a 3-level system architecture is considered for ramp metering. The architecture can consist of the following elements, Figure 64.
  • External facilities: can consist of detectors, video cameras, VMS and information panels, permanent illuminated signs, barriers, traffic signals
  • Local control: local control station with data input/ output devices, connection to power supply and data communication

Area control: control (sub) centre is hierarchically structured and consists of optional control centre, control sub-centre and local control station.

Traffic Monitoring

The devices and methodologies for traffic data collection are not covered by this service description. They depend amongst others on the particular used data collection system and are left to the operator to select; provided a reasonable level of accuracy and reliability is guaranteed. 

Usually there are vehicle detectors on the on-ramp and main carriageway to measures traffic conditions:

• Main carriageway detectors: the location of upstream and downstream detectors depends on the ramp metering algorithm requirements. In some cases, motorways are already equipped with a high density of traffic detectors (i.e. every 500m) or another real-time estimation system providing information on the current main carriageway traffic conditions. If this is the case, no additional detectors are needed.  
• On-ramp detectors: these detectors are needed for queue management and traffic signal operation. For queue management purposes the number and location depends again on the chosen ramp queue management strategy.

Local Controller / Outstation 

The outstation provides control functionality and calculates release rates and the resulting signal timings based on traffic flow.  The outstation can be equipped with a remote communication so that administrative functions can be carried out remotely. 

Ramp metering controllers typically operate in the following states:

• Standby mode – ramp metering lights are switched off
• Activate – activate ramp metering and turn signals on
• Steady state – state of “normal operation”
• Queue override – prevents congestion on the local network, higher release rate
• Deactivate – deactivate ramp metering and turn signals off
• Fail-safe mode – prevents or mitigates unsafe consequences of the system’s failure; depending on the situation this could be switching off or fixed-time control.

The above states are described in many documents, EURAMP Deliverable 7.5 for example. The controller activates the ramp metering lights in accordance with the algorithms and contains an interface to traffic detectors (on the main carriageway and on-ramps). 

The specific combination of switching criteria and threshold values depends on the traffic conditions and control installation, i.e. the system may only be active as long as it is required by the traffic situation; when the traffic situation eases, the system goes into standby mode.

Ramp Metering Algorithms 

Appropriate RM algorithms are used to monitor the traffic conditions and regulate traffic flow on the on-ramp onto the main carriageway. It is recommended that all algorithms are configurable.

Fallback / Failsafe 

If signals or local controller fails, it is recommended that the central coordinated strategy be able to continue its operation and coordination of available ramps taking the missing ramp into account. If there are communication failures between the control system and the local controller should automatically switch to fail-safe mode.

Communications             

Ramp metering systems require power supplies and telecommunication systems such as fibre optic cable or mobile/land-based telephone connections to provide links to the traffic operations centres and the signal controllers; remote communications are becoming more commonly used (sufficient bandwidth is required).

Central Control System 

Operational Graphical User Interface (GUI): this is recommended to allow easy inspection, maintenance and repair of local signal controllers. A traffic operations centre GUI should allow for easy access to parameters, variables and display during operation. At times, analysis of historic variables may be required (i.e. in cases of errors or reconstruction of previous scenarios) and so archiving facilities are advantageous.

Computing Devices

The necessary computing devices may be centralised or decentralised depending on the adopted architecture. It can be of benefit if devices selected are easily scalable and have sufficient computing power to allow for future additions, updates and future strategy changes.

Compliance with EU Delegated Regulations

It is unlikely that the requirements of the EU Delegated regulations for road safety related traffic information and real-time traffic information services will impact greatly on this service. However, if data collected and processed results in safety information, or traffic management information being created which would be of use to the end user, the National Access Point must be used to publish the information.

4.4.3.7 Required standards and specifications

Information provision standards:

• IPS1: If the Ramp Metering service provides ramp metering information at interface 1 (see IFR1), it must be profiled based on EN 16157-3:2019 using the DATEX II Recommended Service Profile for Ramp Metering.
• IPS2: If interface 2 is implemented, Ramp metering information (see IFR2) must be profiled in an SPATEM/MAPEM (Signal Phase And Timing Extended Message/MAP Extended Message) based on ETSI TS 103 301 using the C-ROADS C-ITS Message Profiles for the Traffic Light Manoeuvre service.
• IPS3: When relevant, the Probe Vehicle Data (microscopic traffic situation) information (see IFR3) should be collected, which is profiled based on ETSI EN 302 637-2 using the CAR2CAR Communication Consortium Basic System Profile.

4.4.3.8 Level of Service definition

4.4.3.8.1 Level of Service criteria

Table 30 gives the Level of Service recommendations for a Ramp Metering service. The background of this concept is descripted in chapter 2.6..

Table 30: Level of Service recommendations for Ramp Metering

The levels show technological advancement in the ITS solutions that can be implemented where appropriate; and can be cross-referenced to the Operating Environments.

The Levels of Service described here are not intended to indicate that by deploying Level C at all sites there will be improved results over Level B deployments, i.e. Level B is not “better” that Level A. The Levels only indicate advanced ITS technologies or techniques.

The Level of Service selected is closely related to the operating environment (traffic characteristics, level of incidents, road use etc.). As stated earlier, ramp metering is highly site specific in nature, i.e. Level A deployments may achieve the desire results in certain circumstances; but in other more complex situations another Level of Service may be more appropriate.

Using the table above, implementers can select the level of service of each element that is most appropriate, i.e. Level A Pre-signing with a Level 3 Metering Strategy.

Coverage

• 1: Point coverage – RM is deployed at on-ramps with a specific problem junction 
• 2: Section coverage – RM is deployed at several junction on-ramps on a section 
• 3: Wider route coverage – RM is deployed at on-ramps over longer routes where there are several  bottlenecks / critical sections where congestion occurs

Pre-signing 

• 1: Fixed signs – set number of vehicles per green / fixed operation time 
• 2: Rotating prism VMS – allows changes in the number of vehicles per cycle based on current traffic conditions
• 3: VMS – provides maximum flexibility and can also be used to provide additional information to  road users

Metering Strategies

The sophistication and size of a ramp metering system should reflect the amount of desired improvement and existing conditions. Ramp metering strategies can be based on fixed metering rates (historical), real-time data, or predicted traffic demand. Strategies can be implemented to optimise conditions locally or systemwide. Each control mode has an associated hardware configuration. If ramp control is linked at several junctions, there is greater overall equity. Distinguished by their responsiveness to prevailing traffic conditions, metering systems fall into three categories: 

• 1: Fixed Time Operation 
• 2: Local Traffic Responsive Operation 
• 3: Centralised System-Wide Traffic Responsive Operation

4.4.3.8.2 Level of Service Criteria related to Operating Environment

Level of service requirement:

• LoSR1: Given that pre-deployment surveys / evaluations provide the necessary evidence to proceed with the deployment of the ITS-service “Ramp Metering”, the minimum and optimum LoS should respect the Level of Service to Operating Environment mapping table.

Table 31: Level of Service to Operating Environment mapping table (see also chapter  2.5.3 and ANNEX C)